Different Types of Knuckle Hinges

There are more than one type of knuckle hinges that jewelers use for creating their masterpieces. Explore the different types of options for knuckle hinges for your jewelry making work.

One Knuckle Hinge

While this one may seem a little odd at first glance the idea has merit. Linda Chow suggests that a wire inserted through a hole is a type of hinge which by implication makes an normal jump ring inserted into a hole a kind of hinge. There are several instances of wires placed through tubes as a mobile component in goldsmithing. One example is the use of a ‘figure 8’ safety catch which is often used on box catches. The wire ‘8’ has a small knob ball on the end and snaps over a balled up wire to make the safety device function. It is normally used on the side of a box catch.

Another example is a double pin stem system used for the back of a brooch. The pinstem is inserted through a single tube and bent up on each side. This is the same idea that is used in some hair barrettes.

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Another single knuckle hinge may be found on small Native American boxes from the Southwest United States. Tubing is soldered to the top of the box and wires are inserted and bent downwards. Their ends are soldered onto the bottom of the box. Because the solder joins are a little distance from the tube these can be tensioned so as to create a ‘snap’ when the lid of the box closes. Because they are used in multiples they are in fact a kind of two knuckle hinge.

Two Knuckle Hinges

Two knuckle hinges are not normally functional if built using tubing because the end surfaces (the contact places) of the tube ends in the middle of the hinge are not broad enough to provide sufficient structural strength to hold up in use (this is the voice of experience). The reason for this is that the contact surfaces (the ends of the tubes) between the tubes of a hinge are not very broad, and because the hinge itself is flexed and stressed sideways in use, the hinge pin is bent back and forth and will eventually break. It snaps where the knuckles adjoin. This is why you don’t see many two-knuckle hinges in use.

One can however have a functional and durable two part hinge if the bearing surface area is increased, perhaps by using very thick walled tubing (and I mean very very thick walled tubing) or if it has a massive hinge pin. Let’s imagine taking a two-knuckle hinge, and thickening up the tubes, bigger and bigger and bigger, until what we end up with is two pieces of solid sheet metal with a rivet through them; this works, so this is a functional two-knuckle hinge. And the reason it works is that it is structurally sound, because we’ve increased the bearing surfaces in contact with each other. This is then a two part hinge which lasts. It is a single rivet through two pieces of sheet; it swivels and functions as a hinge and the bearing (contact) surfaces of the ‘knuckles’ is really large: the function of the tubing ends is replaced by sheet metal.

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We could, then, if we had tubing with a thick enough wall (to have those wide contact surfaces), use a two-knuckle hinge, but most of the time this is not a particularly useful style. A two knuckle system that works is the traditional forged link one where square stock is forged so that each end goes from broad to narrow thus changing the axis of the hinge pin and allowing the linkage to move well in two directions alternating every two links.

As Chow points out, two-knuckle hinges also work if they are used in multiples-that is, just like you see with most washroom doors in public bathroom stalls. If you have several sets of two-knuckle hinges in a line, so that it becomes a type of multiple-knuckle system, then it works.

Another way of getting a two-knuckle hinge to work by itself is to have a hinge pin with a huge diameter, so that the hinge pin provides the stability necessary for the hinge to work and hold up in use.

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Three knuckle hinges

The standard hinge is three knuckles. Usually this is three equal sized knuckles, primarily for visual reasons as well as strength. You’ll notice that most hinges that are around consist of three knuckles. It is customary to have each knuckle the same size, but you can vary the relative sizes of the knuckles within certain limits.

If, for instance, you have very small outer knuckles, and a large central one, this is quite stable and strong, but if you have a small central one, and very large outer ones, this is not stable, because there’s insufficient contact surface, insufficient soldered area, between the central knuckle and the hinged material to stand up to the stresses of use and it will tear away eventually. If the outer knuckles are smaller the hinge will still work but extremely narrow outer knuckles may be subject to damage as well. You can however err on the side of small outer knuckles without problems much of the time. Such variations in knuckle size are a design choice-at least until lack of function in use intrudes. So, to avoid problems either have them all the same size, or err on the side of a larger central knuckle and smaller outer knuckles.

In systems where the hinge pin is soldered into the outer knuckles it helps if the outer knuckles are fairly small and the middle one proportionally larger.

equal, small center and small outer knuckles, the center figure will not last in use

Multiple knuckles

One can have as many knuckles on a hinge as one wants. Piano hinge for instance is available commercially in long strips many feet long with hundreds of hinge knuckles. It is then cut to the size needed. Multiple knuckle use is usually reflective of hinge size: a very long hinge works well and is relatively easy to make if it has a lot of knuckles. The reasons to choose multiple knuckles include strength (a long hinge with a lot of knuckles is extremely structurally sound) and design choice.

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One can vary the sizes of each knuckle to introduce an interesting design element. There is the option of leaving some of the knuckles out and exposing the hinge pin as a design element-of interest with a larger hinge pin or one of a different color than the knuckles. In a bracelet these gaps might rapidly fill up with skin gunge (repair people will know all too well what I mean) but this version might work well in places not in direct skin contact.

As was discussed in regard to two knuckle hinges one can have a series of two individual knuckle hinges such as you can see on washroom doors and this is a version of a multiple knuckle hinge. Each of those two knuckle units however has an individual hinge pin and you can’t see a pin running between them as in the drawing above.

Knuckles as an exaggeration (mechanism as intent) For design reasons it can be useful to exaggerate the size, shape, texture, wall thickness or number of knuckles. For instance if you don’t want hinges to be clearly standard hinges or you can’t hide them completely then it may be a solution to a design problem to enlarge the hinge tubing thickness and size so the hinge becomes a clear design element in its own right. You can emphasize the actual knuckles, either in thickness, length, or position relative to the hinge, as a design choice. An elongated hinge, a very thick walled hinge and a hinge where the tubing has been pierced; drilled through or sawed are pictured below as well as a hinge Klaus Ullrich designed for a bracelet in the late 1970’s where the hinge was lifted away from the surface of the bracelet some distance. This bracelet moved and folded in a very intriguing manner.

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False knuckles

Sometimes one wishes to have the visual effect of multiple or staggered knuckles as described above but does not want to go to the trouble of soldering on and fitting all those separate little parts. In that case one can use a standard three knuckle hinge and then slice straight into the knuckles with a thin saw blade after soldering them in place but before inserting the hinge pin (watch out for the saw blade wandering away from a straight line cut when you do this!). The hinge then appears to the eye to be a multiple knuckle, varied knuckle length hinge but in fact consists only of the three soldered knuckles with slits in them. This can offer you some interesting design choices. It has to do with constructing something that appears to be more complex than it really is.